A spoked wheel for a bicycle comprises a peripheral crown or rim, a central hub and a plurality of spokes extending between the hub and the rim, the tension of which must preferably be adjustable at the hub and/or at the rim.
To allow tension adjustment, usually a threading is provided at an end of the spokes and a threaded hole attachment is provided at the hub or the rim, respectively. The second end of the spokes can be provided with a second threading or a head, for example rectangular or circular, housed in a suitable seat, for example hooked in a hub flange or inserted in a bayonet coupling at the rim, respectively.
Rims having a channel, with fins where the beads of the tire are coupled and at the base of which a plurality of holes are made, are known. The holes receive a spoke attachment element provided with a hole with a shaped seat or with a threaded hole. The length of the hole threading of the spoke attachment element can be equal to the spoke threading, or else longer to also allow tension adjustment of the spoke by varying the screwing in depth of the spoke in the threaded hole of the spoke attachment element.
The spoke attachment elements are also called “nipples”, if they allow the tension adjustment of the spoke, or else “barrels” if they do not allow this.
Such type of single channel rims have low structural stiffness and therefore greater deformability from lateral and radial loads. Moreover, such single channel rims are not suitable for use with tubeless tires since the coupling channel is not airtight.
To increase structural stiffness, rims having a circumferentially external tire coupling channel and a circumferentially internal spoke attachment chamber are also known. More specifically, such rims are made up of two side walls connected or filleted to one end by a circumferentially internal wall or lower bridge and at an intermediate point by a circumferentially external wall or upper bridge or partition, so as to have a substantially inverted A-shaped cross-section. The circumferentially external portions of the side walls, typically provided with a border, form the tire coupling channel with the upper bridge or partition, whereas the circumferentially internal portions of the side walls form, with the lower bridge and the upper bridge, a spoke attachment chamber.
Instead of a single chamber, two or more chambers can be provided through one or more partition walls, extending substantially parallel to the upper bridge or substantially transverse to it. Each wall (upper bridge, lower bridge, side and partition walls), can also be shaped in various ways, forming an even remarkably complex cross section of the rim.
Rims of this type can be manufactured by casting of carbon fibre (see for example the European patent application of the same Applicant published with No. 1.231.077 A2). Alternatively, such rims are manufactured by extrusion of an aluminum profiled bar. The profiled bar is shaped as a circle typically by calendaring and its terminal ends are joined by welding, for example flash welding or TIG welding, by cementing after insertion of a metallic sleeve, by pin-connection or otherwise.
In this type of rims there is the problem of how to attach spokes in respective spoke attachment seats provided in the spoke attachment chamber(s), normally through the aforementioned spoke attachment elements.
In a first solution, described in U.S. Pat. No. 5,975,646 of Campagnolo, this is carried out by forming a plurality of access openings in the upper bridge, each at a respective spoke attachment seat. Such access openings allow insertion of the spoke attachment elements (nipples or barrels) and of the tool for screwing them onto the spokes or for holding them during the attachment of the spokes. To preserve a suitable structural stiffness of the rim, the upper bridge must however be formed with increase thickness, with a consequent undesirable weight increase of the rim. Moreover, such a rim is not suitable for tubeless tires since the tire coupling channel is not airtight due to the aforementioned access openings.
US patent application US2001/0019222 A1 of Campagnolo, to avoid this and to provide a rim in which the upper bridge has only the hole for the inflation valve for the tire and therefore sufficiently rigid and light, as well as suitable for use with tubeless tires, proposes to use supports having a seat for supporting a respective nipple at least partially outside of the rim section. Each nipple support more specifically comprises a substantially U-shaped bracket having said seat and hung on a pivot extending transversely between a pair of non-threaded holes formed in the side walls of the circumferentially internal chamber, or in flanges protruding from the side walls of the circumferentially internal chamber.
Regarding such a solution, the Applicant observes that, besides the nipples, additional elements are necessary, with consequent additional assembling steps and greater weakness points in the wheel.
Other known solutions for providing a rim in which the upper bridge has just the hole for the inflation valve of the tire and therefore sufficiently rigid and light, as well as suitable for use with tubeless tires, are described in European patent applications EP 0 896 886 A1, EP 1 101 631 A1, EP 1 167 078 A1, all of Mavic S. A. In all three of such documents it is proposed to form the spoke attachment seats as threaded holes in which to screw, from the outside of the chamber, a threaded end of a spoke, a spoke attachment element such as a nipple or a barrel, or an insert in which to insert a nipple or a barrel. Since the thickness of the rim walls must be small for weight reasons, it is necessary to form the threaded holes by material deformation, so that a “chimney” is created in which a threading of the required length can be formed.
Regarding these solutions, the Applicant observes that they are very complex and expensive since forming the threaded holes in the lower bridge or in any case in the walls of the circumferentially internal chamber is a difficult and complex process.
Therefore, a simple and cost-effective solution to manufacture a spoked wheel for a bicycle, sufficiently strong whilst still being light, is still desirable.